When a switch opens and stops the flow of current from an inductive load, the voltage across the inductor load suddenly rises and keeps rising, trying to force current through the open switch. Eventually, the voltage may become high enough to force current through and destroy the switch. The above-described problem often occurs in high current and high voltage applications. Accordingly, to protect the switch a snubber circuit is coupled across the switch.
Referring to FIG. 1, one example of a circuit 10 with a transistor 12, an inductive load 14, and a snubber circuit 16 is illustrated. In the circuit, the collector C of transistor 12 is connected to a -V source and the emitter E of the transistor is connected to a +V source. An inductive load is connected to the emitter of transistor 12. The snubber circuit 16 is connected in parallel with transistor 12 to the emitter E and collector C. A first load 18 and a first diode 20 are connected in series between the emitter E and a +V source, a second diode 22 is also connected in parallel with transistor 12, and a second load 24 is connected between the snubber circuit 16 and the emitter E.
The snubber circuit 16 consists of a snubber capacitor, a snubber resistor 28, and a snubber diode 30. The snubber capacitor 26 has first and second capacitor terminals 32 and 34 with the first capacitor terminal 32 coupled to the emitter E of transistor 12 and the second capacitor terminal 34 connected to one terminal of the snubber resistor 28. The other terminal of the snubber resistor 28 is connected to the collector C of transistor 12. The snubber diode 30 is connected in parallel with the snubber resistor 28.
When transistor 12 is off and thus open, the full load current is blocked from flowing through transistor 12. Instead, the full load current I.sub.load is transferred to the snubber circuit 16 which charges the snubber capacitor 26 and turns the snubber diode 30 on. The snubber diode 30 must be able to temporarily conduct the full load current I.sub.load and must be able to block the system voltage.
When transistor 12 is on and thus closed, the full load current I.sub.load flows through transistor 12. Meanwhile, snubber capacitor 26 discharges through snubber resistor 28 and snubber diode 30 is turned off. Snubber resistor 28 must be able to handle high peak current dissipation and must be small enough to discharge snubber capacitor 26 before the next turn off event.
Although the above-described snubber circuit 16 works, the snubber circuit 16 has some problems and limitations. For example, in high current and high power voltage applications, snubber resistor 28 is not able to withstand some commonly occurring high peak currents and wattage. The size requirement for the current surge and wattage creates layout problems which result in high stray inductance and reduces the effectiveness of the snubber circuit 16.